A cellular wireless network is a communication network comprising one or more cells where each cell has a base station (or cell site) comprising an antenna. The antenna may be a MIMO (Multiple Input Multiple Output) antenna comprising M antenna elements where M is an integer equal to 2 or greater. The term “antenna elements” refer to separate and independently controllable antennas that are part of one structure. In practice, M can be 100, 500 or even 1000 or more; thus the term massive MIMO. A cell is a defined geographical area having a base station assigned thereto so that mobile terminals located within the cell are able to communicate—via the base station—with other mobile terminals within the cell of the base station or with mobile terminals in other cells; a mobile terminal may also communicate with other communication devices of other communication networks (e.g., landline telephones of the Public Switched Telephone Network (PSTN) or the Internet) depending on whether the cellular network is connected to such other networks.
The base station further comprises transmitter, receiver and signal and information processing equipment that convey (i.e., transmit and/or receive) information to mobile terminals and to other equipment (e.g., other base stations) of the cellular network where such processing equipment and other equipment include equipment that control and/or operate the base stations and the entire communication network. Further, the base stations are connected to each other typically via communication links sometimes referred to as back-haul communication links. The base stations use these back-haul links for, inter alia, communicating information to each other or to other equipment of the cellular network where such information are data (e.g., voice, video, graphics or any combination thereof) being conveyed within the network or information relating to the operation and/or control of the cellular network. The back haul links are typically implemented with optical links or other media with which communication channels having relatively large bandwidths can be constructed.
The mobile terminals are user owned and/or controlled transceiver devices (e.g., cellular telephones) that are registered (by their users) to the cellular network for subscription to the network to allow such users to avail themselves of the communication services being provided by the cellular network. The mobile terminals are able to convey (i.e., transmit and/or receive) information in various forms (e.g., voice, data, text, video, graphics or any combination thereof) to other mobile terminals or to other communication devices that are not part of the cellular network (landline phones, for example). The information is conveyed via one or more base stations and equipment of the cellular communication network. A mobile terminal may communicate with a base station of a cell in which the mobile terminal is currently physically located and, at the same time or at a different time, the mobile terminal may communicate with a base station of one or more neighboring cells or other cells of the cellular network.
Certain wireless cellular networks are considered to be TDD (Time Division Duplex) networks whereby uplink transmissions and downlink transmissions occur at different assigned time slots, but both types of transmissions use the same frequency band or bands. Uplink transmissions are transmissions from mobile terminals to base stations. Downlink transmissions are transmissions from base stations to mobile terminals.
The communication channels through which information between the base stations and the mobile terminals is conveyed are the media (air space including structures, objects and obstacles in the pertinent air space) through which or around which signals between any particular mobile terminal and a base station propagate. The signals may also be partially or totally reflected and/or absorbed by the various structures or obstacles. As with any communication network, the information transmitted through a channel is often adversely affected by anomalies in the channel such as noise, interference, phase jitter, frequency translation and other anomalies that typically occur in communication channels.
To achieve proper and effective downlink communications between the base stations and the mobile terminals, each base station first determines the characteristics of the communication channels between it and mobile terminals (in its cell or in other cells) to which it is conveying (i.e., transmitting and/or receiving) information. The base station then uses the channel characteristics associated with a particular mobile terminal to transmit information to the mobile terminal, which may be located in the cell of said base station or in another cell.
The communication channel characteristics are represented by fast fading coefficients (FFC) and slow fading coefficients (SFC). Mobile terminals that have been registered and are subscribers to the cellular networks are provided with at least two types of pilot signals, which they can transmit at network defined time instances to base stations capable of receiving and processing said pilot signals. A registered mobile terminal is a terminal that has entered into a subscription agreement with the owner and/or operator of the network resulting in the network recognizing and allowing the terminal access to the communication network and the services provided thereby. Mobile terminals typically transmit one type of pilot signals (i.e., FFC pilot sequences) that the base stations use to determine fast fading coefficients and another type of pilot signal (SFC) that the base stations use to determine slow fading coefficients.
The base stations and mobile terminals have a priori knowledge of the pilot signals; that is, each pilot signal is one of a network defined set of signals (or a set of signal sequences) with characteristics known to all of the base stations and registered mobile terminals in the cellular network. Some or all of the characteristics (e.g., amplitude, relative phase, frequency) of a transmitted pilot signal received by a base station will have been affected by the media (i.e., channel for that pilot) through which that pilot signal propagated. The pilot signals received by the base stations are processed by said base stations in a well known manner to derive the channel characteristics, viz., slow fading and fast fading coefficients associated with each mobile terminal that transmitted the pilot signals. The fast fading coefficients, as their name suggests, represent channel characteristics that change relatively frequently over time and/or the change has a relatively wide range. A slow fading coefficient between a base station and a mobile terminal represents the transmit signal power attenuation, i.e. it represents the loss of the signal power during signal's propagation from the base station (mobile terminal) to the mobile terminal (the base station). The slow fading coefficients changes relatively slowly over time. They are also approximately constant within a circle with the radius of several meters.
For the FFC pilot sequences, which are used to derive the fast fading coefficients, each is, by definition, a vector containing q components where q is an integer equal to 2 or greater. The term pilot vectors or pilot sequences (or FFC pilot vectors; FFC pilot sequences) are hereinafter used interchangeably with the term FFC pilot signals. The network may thus define a set of K (note that K≦q) (an integer equal to 2 or greater) q-component pilot signals each of which is orthogonal to the other (K−1) q-component pilot signals. Thus, for example, for FFC pilot sequences rk and ru where k≠u and (k, u, =1, . . . , K) and rk, ru each is an FFC pilot signal from the set of K network defined pilot signals with each pilot signal having q components, rk·ru=0; and for u=k, rk=ru and so rk·ru=rk·rk=1. The maximum possible number of orthogonal q-tuples is q. Therefore we have that only K≦q orthogonal FFC pilots can be generated. Because of this and because the pilot signals are transmitted synchronously by various mobile terminals throughout the network, the transmission of non-orthogonal (e.g., identical pilot signals) FFC pilot sequences by different mobiles in different cells (neighboring cells, for example) at the same time does occur resulting in the pilot signals interfering with each other; this phenomenon is referred to as pilot contamination. Consequently, the fast fading coefficients derived from the transmitted non-orthogonal FFC pilot sequences will contain errors. The fast fading coefficients between a given base station and a given mobile terminal form an M-dimensional vector, since each of the M antennas has its own fast fading coefficient. This vector is called fast fading coefficient vector.
The slow fading coefficients estimated with the use of SFC pilot signals are not affected by this pilot contamination problem and are not affected by the channel anomalies as much, due to their relatively stable and slow varying characteristics, and because these coefficients are derived using a different and more numerous set of pilot signals. Each base station uses FFC pilot sequences to estimate the fast fading coefficient vectors (also referred to as Channel State Information, i.e., CSI) between it and the mobile terminals located in the cell of the base stations. The base stations use their CSI estimates to process uplink signals from the mobile terminals located in the cells of the base stations. The CSI estimates are also used in downlink transmissions from base stations to corresponding mobile units. The data for this downlink transmission is received by base stations from back-haul lines.
TDD wireless networks achieve relatively high data transmission rates through their uplinks and downlinks. However, several problems prevent such networks from achieving even higher data transmission rates—especially for downlink transmissions. The problems are due to interference caused by the pilot contamination issue discussed above where the reuse of the same FFC pilot vectors (i.e., FFC pilot sequences) is unavoidable, and to other sources of interference. The interference resulting from two or more synchronously transmitted identical (or at least nonorthogonal) pilot signal sequences is referred to as directed inter-cell interference. Because the same FFC pilot sequence is used by several mobile terminals the estimates of the fast fading coefficient vectors are corrupted. A base station uses these corrupted estimates to generate M-dimensional beamforming vectors that point not only to the mobile terminal located in the cell of the base station, but also to terminals located in the neighboring cells; this results in the directed inter-cell interference.
As a result of the directed inter-cell interference, several problematic issues arise: (i) the directed inter-cell interference does not disappear even as M (i.e., the number of antenna elements per base station) tends to infinity; (ii) estimation errors in the estimates of the fast fading coefficient vectors are also due to additive noise in the hardware of base station receivers; these estimation errors corrupt the beamforming vectors, which are part of the downlink signals transmitted by the base stations. This results in indirect intra-cell and inter-cell interferences; (iii) As M—the number of antenna elements at each base station—increases and tends to infinity, the fast fading coefficient vectors (or CSI) become mutually orthogonal; however, because M is finite, the vectors are not orthogonal and this causes additional indirect inter-cell and inter-cell interferences; (iv) mobile terminals do not know the effective channel gain between themselves and a base stations (i.e., the amount of signal amplification needed to be done by a mobile terminals in transmitting information to the base station of the cell in which the mobile terminal is physically located); because the mobile terminals have to estimate such a gain, the error due to estimation reduces the Signal to Interference Noise Ratio (SINR) of the downlinks.
As the number of antennas M, increases, the interferences in (ii), (iii) and (iv) above get progressively smaller. As M approaches infinity, the interferences due to (ii), (iii) and (iv) approach zero. However, in sharp contrast, the directed inter-cell interference caused by the pilot contamination problem increases with M. As such, even with an infinitely large M, the directed inter-cell interference becomes the main and only reason for a lack of increase in the SINR to relatively large values. Thus, the directed inter-cell interference resulting from pilot contamination prevents the SINR from increasing even with increasing M and consequently inhibits the network from increasing its downlink transmission rates.